What pokemon are you using? What pokemon are you facing? What moves do your pokemon know? Did you buy your game in a different country? Did you buy your ds in a different country? If so, where? Is the game used? Have you contacted nintendo? Is there other people on the internet with this problem?
You should look at all these things and see if you can fix your game. If your game is still having problems, try creating a new file. If you can use moves on that file then you have to erase your game and beat it again. If this doesn't work, then please go to where you bought your game and get a refund. These are the only ways you can get your game to work.